The old lady was wearing her beautiful, fancy glasses that she wore just to look great.  The problem was that she couldn't actually see anything through them (she might as well have been looking through stove-lids, which are round pieces of black cast iron).  Apparently she had bought the glasses for their looks at a used goods store -- they clearly weren't custom made for her vision needs.

There are two ways to interpret the phrase, "She seldom or never looked through them for so small a thing as a boy."

One is that the only things she could see through her glasses were things that were physically much larger than a boy. So, if she wanted to find a small boy, she had to look above or below the glasses not through them.

The other is that she didn't care what the boy thought of her, so she didn't mind looking over or under and actually being able to see something.  She only suffered through the effort of trying to see through them for more important people than a boy.